Pneumatic tyre

Abstract: A pneumatic tyre (1) comprising a tread (2) having a plurality of main grooves (3) extending continually in the circumferential direction and a plurality of lateral grooves (4) crossing between the adjoining main grooves (3) and having a groove depth DB of 0.6 to 0.95 times the mean DM of groove depths DA1, DA2 of the adjoining main grooves (3), characterised in that said plurality of lateral grooves (4) are composed of, in a section orthogonal to the groove centre line, an upper portion (6) having an opening at the tread surface (2A), a lower portion (9) having a groove bottom and a constricted waist portion (7) consecutive to the upper and lower portions (6,9) and having the narrowest width WS of each lateral groove, and the following relation is established: 1.0mm ≦ WS ≦ 4.0mm 1.3 ≦ WW/WS ≦ 3.0 0.15 ≦ DS/DB ≦ 0.7 where: WS is the narrowest width, WW is the widest width of the lower portion, DS is the length of the waist portion in the tyre radial direction.

Description: The present invention relates to a pneumatic tyre capable of preventing heel and toe wear on the block surface, and maintaining wet grip performance and wear appearance for a long period. In high speed running on a wet road surface, hitherto, in order to improve the water discharging performance and suppress the phenomenon of hydroplaning, the tread surface was provided with a plurality of main grooves extending in the circumferential direction, and multiple lateral grooves linking with the longitudinal grooves. Such lateral grooves have been designed to effectively suppress the hydroplaning and maintain appearance even at the terminal stage of the tread wear by for example the method shown in Fig. 11, which has (1) Deepened lateral grooves groove depth DB. (2) Widened lateral grooves WO'. (3) Having the sectional shape of the lateral grooves in a U form. However, when the lateral grooves are thus constituted, when running on a dry road, uneven wear along the groove edge of the lateral grooves, or so-called heel and toe wear occurs, and the durability is lowered. It is hence the object of the invention to present a pneumatic tyre capable of preventing heel and toe wear, maintaining the wet grip performance and wear appearance for a long period, and enhancing the running performance also on a snow-clad road. The present inventor has reached the invention by discovering that: (1) The sectional shape of the lateral grooves should not be uniform in groove width, but a middle pattern of the groove in the depth direction should be provided with a waist portion constricted in width so as to make the upper and lower portions wider than this constricted waist portion. (2) By defining the groove width ratio of the lower portion and constricted waist portion, the wet grip performance and tyre appearance can be maintained even in the terminal stage of wear. (3) By defining further the ratio of the length of the constricted waist portion and the depth of the lateral groove, the wet grip performance is enhanced. According to the present invention, a pneumatic tyre comprises a tread having a plurality of main grooves extending continually in the circumferential direction and a plurality of lateral grooves crossing between the adjoining main grooves and having a groove depth DB of 0.6 to 0.95 times the mean DM of groove depths DA1, DA2 of the adjoining main grooves, characterised in that said plurality of lateral grooves are composed of, in a section orthogonal to the groove centre line, an upper portion having an opening at the tread surface, a lower portion having a groove bottom and a constricted waist portion consecutive to the upper and lower portions and having the narrowest width WS of each lateral groove, and the following relation is established: 1.0mm ≦ WS ≦ 4.0mm 1.3 ≦ WW/WS ≦ 3.0 0.15 ≦ DS/DB ≦ 0.7 where: WS is the narrowest width, WW is the widest width of the lower portion, DS is the length of the waist portion in the tyre radial direction. The opening width WO of the lateral grooves is preferably 1.5 to 6.0 times the narrowest width WS. The groove depth DB of the lateral groove is set at 0.6 to 0.95 times the mean DM of the groove depth of the main grooves at its both sides. If the groove depth DB is less than 0.6 times the mean DM, the lateral groove is eliminated before the tread reaches its wear life. As a result, the wet grip performance is not maintained for a long time, and the tyre appearance is spoiled. If exceeding 0.95 times, to the contrary, the pattern rigidity is lowered, and uneven wear of heel and toe type occurs early at the groove edge of the lateral groove. In the constricted waist portion of the lateral groove, the narrowest width WS is defined as 1.0mm to 4.0mm. If the width WS is less than 1.0mm, the water on the tread surface is blocked in the constricted waist portion and cannot reach the lower portion. As a result, the water discharging performance is inferior, thereby lowering the wet frictional force. To the contrary, if exceeding 4.0mm, when contacting with the ground, if the block is deformed by static or dynamic load, the opposing groove walls do not contact with each other, and hence the pattern rigidity is insufficient, and early heel and toe wear occurs. The ratio DS/DB of the length DS of the constricted waist portion to the lateral groove depth DB is defined as 0.15 to 0.7. If the ratio DS/DB is less than 0.15, the pattern rigidity is insufficient, and heel and toe wear is likely to occur. If exceeding 0.7, since the groove area of the upper and lower portion cannot be maintained sufficiently, water cannot be discharged effectively so as to lower the water discharging performance. More preferably, the ratio DS/DB is in the range of 0.2 to 0.3. These relations have been proved by the experiments mentioned later, and test data are shown in Fig. 6. The lateral groove can efficiently guide the water into the main grooves from the lower portion wider than the constricted waist portion, thereby enhancing the wet frictional force. Still more, unlike the conventional tyre, it is not necessary to form multiple sipings, and the pattern rigidity is maintained, and uneven wear such as heel and toe wear can be suppressed. If the ratio WW/WS is less than 1.3, the wet frictional force drops, or when exceeding 3.0, the pattern rigidity declines and uneven wear is likely to occur. Moreover when exceeding 3.0, it is hard to separate the tyre from the mould after vulcanising, and rubber defect is likely to occur on demoulding. At the boundary of the ratio WW/WS (=1.3), as shown in Fig. 7, it has been experimentally proved that the wet frictional force changes significantly. In the invention, these constituent elements are organically bonded and unified, and the wet frictional force and wear appearance can be maintained until the final stage of wear while preventing the occurrence of heel and toe wear. An embodiment of the present invention will now be described, by way of example only, referring to the attached diagrammatic drawings, in which: Fig. 1 is a developed plan view showing a pattern of a tyre embodiment of the invention; Fig. 2 is an axial sectional view of the tyre; Fig. 3 is a sectional view along line X-X showing an example of lateral grooves; Fig. 4 is a sectional view showing another embodiment of lateral grooves; Fig. 5 is a sectional view showing yet another embodiment of lateral grooves; Fig. 6 is a sectional view showing still another embodiment of lateral grooves; Fig. 7 is a graph showing the relation between the ratio WW/WS of the lateral grooves and the wet frictional force; Fig. 8 is a graph showing the relation between the ratio WO/WS of the lateral grooves and the wet frictional force; Fig. 9 is a graph showing the amount of tread surface wear against the ratio DS/DB and the narrowest width WS; Fig. 10 is a graph showing the relation between the widest groove width WW3 and the wet frictional force when the lateral grooves have a remaining depth of 3mm; Fig. 11 is a sectional view showing lateral grooves in a conventional tyre; and Fig. 12 is a sectional view showing lateral grooves in a comparative tyre of Table 1. A pneumatic tyre 1 comprises a tread 2 with a tread pattern, two side walls 13 each extending inwardly in the tyre's radial direction from both edges of the tread 2 and two beads 14 one located at the inward end of each sidewall 13. The pneumatic tyre 1 is, also, reinforced by a toroidal carcass 16 extending from the tread 2 through the side walls 13 and turned up around the bead core 15 of the bead 14, a belt layer 17 disposed radially outside the carcass 16, and a bead apex rubber 18 extending radially outwardly from each bead core 15 between the carcass main body and carcass turned up portions. The carcass 16 is composed of one carcass ply of steel cords in this embodiment. The carcass ply has the carcass cords arranged at an angle of 80 to 90 degrees, in this embodiment, to the tyre equator C and covered with topping rubber. Nylon, polyester, rayon, aromatic polyamide, or other organic fibre cords may also be used as carcass cords. The belt layer 17 is composed of two to four belt plies, four belt plies 17A in this embodiment. The belt plies 17A comprise belt cords such as nylon, polyester, rayon, aromatic polyamide, or other organic fibre cords or steel cords, which are arranged in mutually intersecting directions between plies. The tread 2 has a plurality of main grooves 3 extending continually in the circumferential direction and a plurality of lateral grooves 4 crossing between the adjoining main grooves 3 in the axial direction of the tyre. The main grooves 3 comprise, in this embodiment, a pair of zigzag main inner grooves 3A folded in, disposed at either side of the tyre equator C, and a pair of outer main grooves 3B each interposed between one inner main groove 3A and its tread edge E, in a linear form to provide alternately disposing broad and narrow portions. The inner and outer main grooves 3A, 3B are of trapezoid sectional shape having a maximum groove width at the opening on the tread surface, as shown in Fig. 2. The maximum groove width of these inner and outer main grooves 3A, 3B is set in a range of 0.06 to 0.12 times the tread width WT, and the groove depths DA1, DA2 are set in a range of 0.08 to 0.16 times the tread width WT. The lateral grooves 4 comprise middle lateral grooves 4A crossing between the inner main grooves 3A and outer lateral grooves 4B crossing between the inner main groove 3A and the outer main groove 3B. In this embodiment lug grooves 20 extending from the outer main groove 3B to the tread edge E are provided. The middle lateral groove 4A is in a continuous Z-form composed of two lateral groove elements 22, 23 extending from one and other inner main grooves 3A, 3B in the tyre axial direction and connecting groove elements 21 between the lateral groove elements 22, 23 in the circumferential direction near the tyre equator C. Therefore, the adjoining middle main grooves 3A, 3B are mutually coupled with the groove elements 21, 22, 23. The outer lateral grooves 4B linearly couple the inner and outer main grooves 3A, 3B substantially parallel in the tyre axial direction. The outer lateral groove 4B has a gourd-shape, as shown in Fig. 3, on a section orthogonal to the groove centre line, such that the middle of the groove in the depth direction is provided with a waist portion 7 constricted in width. That is, the outer lateral groove 4B consists of an upper portion 6 which opens to the tread surface 2A, a constricted waist portion 7 which is adjacent to the radially inner end of the upper portion 7 and has a narrowest width WS of the lateral groove 4B and a lower portion 9 which is adjacent to the radially inner end of the constricted waist portion 7 and has a groove bottom. Thus, the upper and lower portions 6, 9 are formed wider than the constricted waist portion 7. Also, the upper, waist and lower portions 6, 7, 9 are distinguished by a bending point or an inflection point at which the profiles of the upper, waist and lower portions 6, 7, 9 join. In this embodiment, the upper, waist and lower portions 6, 7, 9 join at an edge, respectively. The groove depth DB of the outer lateral groove 4B is set at 0.6 to 0.95 times the mean DM of the groove depths DA1, DA2 of the inner and outer main grooves 3A, 3B. The narrowest width WS of the outer lateral groove 4B is set at 1.0mm to 4.0mm in ordinary automotive tyres, such as tyres for passenger car, small truck and truck-bus, except for tyres for special vehicles such as off-road vehicles and agricultural vehicles. As mentioned above, it is supposed that the opposing groove walls of the lateral grooves contact with each other in the normal ground contact state. Therefore, in the tyres for the designated vehicles, the effect for contacting opposing groove walls is not so much influenced by the tyre size. In the embodiment, the opposing groove walls in the constricted waist portion 7 are parallel to each other so as to have an equal width over the entire region of the constricted waist portion 7. The ratio DS/DB of the length DS in the tyre radial direction of the constricted waist portion 7 to the groove depth DB of the outer lateral groove 4B is set at 0.15 to 0.7. The lower portion 9 is a circular shape like a bottom of a flask, and its maximum groove width WW is set at 1.3 to 3.0 times the narrowest width WS. The upper portion 6 is formed, in this embodiment, like a funnel with a wide opening. The opposing groove walls in the upper portion 6 are inclined at an angle  so that water may be easily guided from the ground to the lateral grooves 4. Since the constricted waist portion 7 achieves the effect for contacting the opposing groove walls to enhance the rigidity, even if the upper portion 6 is formed wider than the constricted waist portion 7, uneven wear does not occur. In the embodiment, moreover, the ratio WO/WS of the groove width WO at the opening on the tread surface 2A to the narrowest width WS is set at 1.5 to 6.0. By thus defining the ratio WO/WS, enhancement of guiding efficiency of water into the constricted waist portion and maintenance of wear resistance are both achieved. By setting the mean groove width of the upper portion 6 larger than 1.5 times, or more preferably larger than 2.5 times the narrowest width WS, or by setting the angle  larger than 15 degrees, or more preferably larger than 25 degrees, the guiding efficiency of water into the constricted waist portion 7 may be further enhanced. The above mean groove width of the upper portion 6 is the mean of the groove widths at radially upper and lower ends of the upper portion 6. If the ratio WO/WS is less than 1.5, the opening width WO is too narrow, and the water is not efficiently guided to the lateral groove 4B. If the ratio WO/WS is set larger than 6.0, the effect of guiding the water is not improved so much, and since the ground contact area decreases too much, the wear resistance of the tread is lowered. Considering these reasons, the ratio WO/WS is preferred to be 1.5 to 6.0, and more preferably in a range of 2.5 to 5.0. The middle lateral groove 4A has a groove sectional shape nearly the same as the outer lateral groove 4B, and has the upper portion 6, the constricted waist portion 7 and the lower portion 9 the same construction as above. The lug groove 20 is mostly out of the ground contact area in the normal ground contact state, and is hence formed with the same U-section as the ordinary one considering only water discharging performance. Other forms of lateral grooves 4 are shown in Figs. 4, 5 and 6. In Fig. 4, the lower portion 9 is formed in a trapezoidal form gradually increasing radially inwardly in its groove width. The constricted waist portion 7 is provided with a convex part 7b wider in width at the radially middle position. Above and below the concave part 7b, an upper part 7a and a lower part 7c are provided extending to the same width. The sum (DS1 + DS2) of the lengths of the upper part 7a and lower part 7c in the radial direction is preferred to be 1/2 or more times the overall length DS of the constricted waist portion 7. If less than 1/2 times, the pattern rigidity is insufficient, and uneven wear may occur. More preferably, it should be kept 2/3 times or more. Moreover, the constricted waist portion 7 may be formed in a taper decreasing in width radially inwardly as shown in Fig. 5, or as a reverse taper increasing in width radially inwardly. In this case, the groove width of the constricted waist portion 7 is preferred to be 4.0mm or less along the entire region of the constricted waist portion 7, more preferably 3.0mm. To prevent local concentration of the stress when the opposing walls contact to each other, the taper angle α of the opposing walls may be set at 6 degrees or less. However, if the groove width of the constricted waist portion 7 is uneven as shown in Fig. 4 or 5, as mentioned above, there is a risk of concentration of stress in part of the constricted waist portion 7. Therefore, the constricted waist portion 7 should be more preferably formed so that the opposing groove walls may be parallel to each other as shown in Fig. 3 or Fig. 6. Example tyres were made to illustrate the invention as follows: A) Test 1 In the tyres of which size is 11R 22.5, constituted as shown in Figs. 1 to 3, the ratio WS/WW was varied, and the frictional force when running on a wet road was evaluated. 1) Frictional force when running on wet road (wet frictional force): Test tyres were installed on a 10-wheel truck, which was loaded as specified and driven on an asphalt road at a speed of 60 km/h. Only the front tyres were braked by locking, and the speed of recovering the frictional force was measured. The result of the test is shown in Fig. 7 by an index with the value at the ratio WS/WW of 1.0 as 100. The greater figure means the better performance. A favourable wet frictional force is recognised by setting the ratio WS/WW at 1.3 or more,. B) Test 2 Using tyres of the same size as in test 1, the ratio WO/WS was varied, and the frictional force running on a wet road was evaluated. Testing was at the same condition as in test 1, and the result is indicated in Fig. 8 by the index with the value of the ratio WO/WS of 1.0 as 100. The wet frictional force was increased by setting the ratio WO/WS at 1.5 or more, but when exceeding 6.0, the frictional force tended to be stable in a high range. C) Test 3 Using tyres of the same size and constitution as in test 1, changes of heel and toe uneven wear were investigated by changing the narrowest width WS and the ratio DS/DB. In the test method, using the same vehicle as in test 1, after running for 30,000 km, heel and toe wear was measured. The test result is shown in Fig. 9. The symbols in the diagram show the magnitude of wear recorded in Table 1. As a result of the test, it is confirmed that the resistance to uneven wear is excellent when the narrowest width WS is in a range of 1.0 to 4.0mm and the ratio DS/DB is 0.15 or more. D) Test 4 Using tyres of the same size and constitution as in test 1, after trial run, when the lateral grooves 4 were worn to leave 3mm in depth, the relation between the groove width WW3 in the lower portion 9 and the frictional force of running on wet road was investigated. Before the trial run, the ratio WW/WS was 1.3. The test method conforms to test 1, and the wet frictional force is expressed as an index with the value of WW3 of 1.3mm as 100. The greater figure means the better performance. The test result is shown in Fig. 10. As known from the result, in the terminal stage of wear of the remaining groove of 3mm, it was confirmed that the wet frictional force is maintained as far as the groove width of the lower portion 9 is 1.3mm. E) Test 5 In the tyres of the same size and same constitution as in test 1, the tyres having the lateral grooves of the invention (embodiments) and tyres having the lateral grooves in Fig. 11 or 12 (comparisons 1 to 3) were compared in wet frictional force, heel and toe wear amount, and wet frictional force (in the remaining groove of 3mm). The wet frictional force conformed to test 1, the heel and toe wear, test 3, and the wet frictional force in the remaining groove of 3mm, test 4. The index is shown with comparison 2 taken as 100, and the greater figure means a better result. The test results are shown in Table 2. As a result of the tests, as compared with the comparisons, the embodiments were confirmed to be enhanced in wet frictional force, heel and toe wear amount, and wet frictional force in the remaining groove of 3mm, in good balance, and were recognised to have achieved the object. Symbol Wear amount o ○ Less than 1.0 mm ○ 1.0 to 1.5 mm ▵ 1.5 to 2.0 mm X 2.0 to 2.5 mm XX More than 2.5 mm

Claims: A pneumatic tyre (1) comprising a tread (2) having a plurality of main grooves (3) extending continually in the circumferential direction and a plurality of lateral grooves (4) crossing between the adjoining main grooves (3) and having a groove depth DB of 0.6 to 0.95 times the mean DM of groove depths DA1, DA2 of the adjoining main grooves (3), characterised in that said plurality of lateral grooves (4) are composed of, in a section orthogonal to the groove centre line, an upper portion (6) having an opening at the tread surface (2A), a lower portion (9) having a groove bottom and a constricted waist portion (7) consecutive to the upper and lower portions (6,9) and having the narrowest width WS of each lateral groove, and the following relation is established: 1.0mm ≦ WS ≦ 4.0mm 1.3 ≦ WW/WS ≦ 3.0 0.15 ≦ DS/DB ≦ 0.7 where: WS is the narrowest width, WW is the widest width of the lower portion, DS is the length of the waist portion in the tyre radial direction. A pneumatic tyre according to claim 1, characterised in that the plurality of lateral grooves (4) have a ratio WO/WS of the opening width WO on the tread surface (2A) to the narrowest width WS defined in a range of 1.5 to 6.0.

Inventors: FUJIWARA KENICHI, MINAMI NOBUAKI, FUJIWARA, KENICHI, MINAMI, NOBUAKI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1994
Main classification: B60C11
IPC codes: B60C11